The present invention relates to a ridge waveguide (RWG) semiconductor laser diode having increased output power, to a distributed feedback (DFB) RWG semiconductor laser diode of this kind which exhibits dynamic single longitudinal mode along with increased output power, and, more particularly, to a high power RWG semiconductor laser diode, such as a DFB RWG semiconductor laser diode, having reduced antiguiding effects within the waveguide which permits a larger single mode guide to be utilized.
High efficiency, high power lasers have long been pursued for such applications as optical pumping of solid state and fiber lasers, direct material processing, printing, communications, sensing, etc. For example, U.S. Pat. No. 5,818,860, entitled High Power Semiconductor Laser Diode, assigned to David Sarnoff Research Center, Inc., describes a broadened-waveguide technique for producing high-power DFB lasers.
The broadened waveguide concept described in U.S. Pat. No. 5,818,860 permits low loss and therefore high-power lasing in multimode sources. Other characteristics inherent in this concept are of particular promise for high-power single-spatial-mode and dynamic-single-longitudinal-mode lasing. Results of an initial attempt in which the broadened waveguide was incorporated into a 1.55 xcexcm single mode DFB RWG diode laser has provided encouragement; as there was attained a 200 mW power output single mode, xe2x88x92165 dBm/Hz RIN from 0 to 2 GHz, and 200 kHz linewidths for 1.5 mm cavity length implementations. Further, FIG. 1 shows the RIN performance achieved and 300 MHz linewidth with a broadened waveguide DFB laser. As shown in FIG. 2, this laser emitted 200 mW cw at 1.55 xcexcm wavelength.
High-power ridge waveguide (RWG) lasers use a cold-cavity index, i.e., effective index, stepof xcx9cxcex94n=0.01, but this value under current injection is diminished by antiguiding. Although antiguiding is quantitatively difficult to estimate accurately and is variable, proprietary experiments and extensive published accounts of conventional RWG laser structures lead one to conclude that latitude in the choice of xcex94n is severely compromised by the antiguiding phenomenon. As a result, xcex94n must be designed to substantially exceed the maximum anticipated antiguiding diminution. For RWG lasers of the prior art, antiguiding has required xcex94n values so great that ridge widths must be limited to xcx9c3.5 xcexcm or narrower to attain a stable, single waveguide mode. The restriction in ridge width limits the power that can be achieved by the laser for several reasons: Firstly, the maximum current density that can be usefully pumped into a semiconductor active region may be limited by phenomena such as the maximum attainable conduction band offsets or other phenomena which affect the maximum power attainable. For wider ridges, a greater current per unit length can usefully be pumped into the active region, causing higher powers to be emitted. Such effects limit the maximum power emitted by the RWG laser under both cw and pulsed conditions. Secondly, an increased ridge width would provide a greater surface area for heat dissipation. Since laser diode performance is severely restricted as temperature rises, wider ridges would permit greater currents to be pumped into the RWG laser and greater powers to be emitted. Such effects presently limit the maximum power emitted by a RWG laser under cw conditions.
Therefore, a high-power RWG laser having a ridge width greater than xcx9c3.5 xcexcm is needed to provide further gains in power output from any RWG laser such as a DFB RWG laser.
A semiconductor laser diode comprises a body of a semiconductor material having a length of at least substantially 3 millimeters; a low-propagation-loss waveguide region formed in the body, having a thickness of at least 500 nanometers; a ridge structure disposed over a side of the waveguide region. For applications requiring dynamic single-longitudinal-mode operation, the diode also includes a distributed feedback structure associated with at least one of the waveguide region and ridge structure. The effective refractive index difference between the ridge structure and exposed portions of the waveguide region which surround the ridge structure is less than 0.003. Accordingly, the width of the ridge can be expanded beyond 3.5 microns.